1. Field of the Invention
This invention relates generally to centrifuge rotors made from composite materials, and relates more particularly to a process of fabricating structures, including centrifuge rotors, by resin transfer molding, and the resulting structures or rotors.
2. Description of the Relevant Art
Centrifuges are commonly used in medical and biological research for separating and purifying materials of differing densities. A centrifuge includes a rotor typically capable of spinning at tens of thousands of revolutions per minute.
A preparative centrifuge rotor has some means for accepting tubes or bottles containing the samples to be centrifuged. Preparative rotors are commonly classified according to the orientation of the sample tubes or bottles. Vertical tube rotors carry the sample tubes or bottles in a vertical orientation, parallel to the vertical rotor axis. Fixed-angle rotors carry the sample tubes or bottles at an angle inclined with respect to the rotor axis, with the bottoms of the sample tubes being inclined away from the rotor axis so that centrifugal force during centrifugation forces the sample toward the bottom of the sample tube or bottle. Swinging bucket rotors have pivoting tube carriers that are upright when the rotor is stopped and that pivot the bottoms of the tubes outward under centrifugal force.
Many centrifuge rotors are fabricated from metal. Since weight is a concern, titanium and aluminum are commonly used materials for metal centrifuge rotors.
Fiber-reinforced, composite structures have also been used for centrifuge rotors. Composite centrifuge rotors are typically made from laminated layers of carbon fibers embedded in an epoxy resin matrix. The fibers are arranged in multiple layers extending in varying directions at right angles to the rotor axis. During fabrication of such a rotor, the carbon fibers and resin matrix are cured under high pressure and temperature to produce a very strong but lightweight rotor. U.S. Pat. Nos. 4,781,669 and 4,790,808 are examples of this type of construction.
Composite centrifuge rotors are stronger and lighter than equivalent metal rotors, being perhaps 60% lighter than titanium and 40% lighter than aluminum rotors of equivalent size. The lighter weight of a composite rotor translates into a much smaller mass moment of inertia than that of a comparable metal rotor. The smaller moment of inertia of a composite rotor reduces acceleration and deceleration times of a centrifugation process, thereby resulting in quicker centrifugation runs. In addition, a composite rotor reduces the loads on the centrifugal drive unit as compared to an equivalent metal rotor, so that the motor driving the centrifuge will last longer. Composite rotors also have the advantage of lower kinetic energy than metal rotors due to the smaller mass moment of inertia for the same rotational speed, which reduces centrifuge damage in case of rotor failure. The materials used in composite rotors are resistant to corrosion against many solvents used in centrifugation.
A disadvantage of composite centrifuge rotors is that the loading of the rotor due to centrifugal forces can cause delaminations and failure of the structure. Reinforcing structures such as outer shells may be necessary to provide adequate structural strength, such as disclosed in U.S. Pat. Nos. 5,362,301 and 4,790,808. Another disadvantage is that extensive and costly machining of the laminated core is required in order to form the outer shape of the rotor and to form the cell holes that receive the sample tubes or bottles containing the samples to be centrifuged.